1. Field of the Invention
The present invention relates to a process for preparing or modifying a hydrogel (a gel containing water) by freezing and then thawing an aqueous solution of a polyvinyl alcohol, and more particularly to a process for improving mechanical strengths of a hydrogel made of a frozen polyvinyl alcohol gel.
2. Prior Art
The process for the preparation of a hydrogel comprising the step of freezing an aqueous solution of polyvinyl alcohol optionally added with an alcohol followed by thawing has been well-known in the art, for example, by Japanese Patent Publication No. 12854/1972 and U.S. Pat. No. 3,875,302 (1975). However, the gel prepared by this known process is too soft and weak and is continuously swollen for a long time to become far softer when it is dipped in water. Particularly, a gel prepared from a polyvinyl alcohol having a relatively low degree of hydrolysis or low polymerization degree or prepared from a starting aqueous solution which is low in concentration of polyvinyl alcohol, becomes so softer as to collapse to lose its integrity within a short period of time when it is dipped in water. The known gel, i.e. the hydrogel prepared through the step of freezing and subsequent thawing, has disadvantages that it is too soft and adhesive and tends to become softer by swelling, as described above, and these disadvantages are the common problems of the gels prepared from commercially available polyvinyl alcohols irrespective of the degree of hydrolysis, polymerization degree and concentration thereof. Because of these disadvantages, the prior art hydrogels of polyvinyl alcohol can be used only for limited applications, such as artificial bait, in which the softness and swelling tendency in water do not raise serious problems.
An antecedent proposal for improving mechanical strengths of a hydrogel of polyvinyl alcohol has been made by one of the inventors of this invention and is disclosed in copending U.S. application Ser. No. 400,821, filed on July 22, 1982, now abandoned. The proposal resides in the provision of a process for the preparation of a gel for use as a cooling medium, comprising the steps of casting an aqueous solution containing a polyvinyl alcohol having a degree of hydrolysis of not less than 95 mol% and a viscosity average polymerization degree of not less than 1500 into a mold having desired shape and dimensions, cooling the cast aqueous solution to a temperature of not higher than -6.degree. C., and then dehydrating in vacuum until the percentage dehydration rate reaches not less than 5 wt%. The process of this prior-made proposal indispensably involves the step of dehydrating in vacuum, and thus there are some disadvantages that dehydration of the hydrogel is not thoroughgoing enough due to small exposed surface area when it is desired to form a mass or article of special shape. For instance, when it is desired to mold an elongated pipe from the hydrogel of polyvinyl alcohol according to the known process including the step of dehydration in vacuum, an aqueous solution of a polyvinyl alcohol is cast in a mold of concentric double pipe and subjected to dehydration in vacuum after being cooled. However, since the exposed surface areas of the mass to be dehydrated are only the small end edges of the cylindrical mass, effect of vacuum dehydration is not exerted deeply enough into the internal portions of the mass, leading to unsatisfactory strength of the product hydrogel.
Hydrogels are, in general, expected as extremely favourable materials for medical uses, since they cause little damage on living tissues and are high in permeability to various substances and improved in anti-thrombotic property with the increase in water content. However, serious problems of inferior mechanical strengths of the product hydrogel in water and of less hardness of the product hydrogel hinder comprehensive uses thereof for medical materials.